Lesson Plan

• Describe the concept of a gravitational field and its mathematical expression.
• Explain the rules for drawing gravitational field lines and how line density relates to field strength.
• Sketch field‑line diagrams for a point mass, a binary system, and near Earth’s surface.
• Apply Gauss’s law for gravity using field‑line representations to calculate flux.
• Identify common misconceptions about field lines.

• Projector or interactive whiteboard
• Slides/handout on gravitational fields and field‑line rules
• Whiteboard and markers
• Worksheet with sketching and calculation tasks
• Printable mass and distance cards for binary‑mass activity
• Rulers and compasses for accurate diagrams

Begin with a quick visual of planets orbiting and ask students how we can “see” the invisible pull of gravity. Recall the definition of a gravitational field from previous lessons and state that today they will learn to represent this field with lines. Success will be measured by accurate sketches and correct explanations of line density.

1. Do‑now (5 min): Mini‑whiteboard question – “What is the formula for gravitational field strength?” Students write answers and share.
2. Mini‑lecture (10 min): Review $g = GM/r^{2}$ and introduce the four rules for gravitational field lines using slides.
3. Guided practice (12 min): Whole‑class sketch of field lines for a single point mass, discussing direction and density.
4. Group activity (15 min): Using mass cards, groups draw the field‑line diagram for two equal masses, locate the zero‑field point, and present.
5. Application (8 min): Demonstrate Gauss’s law for gravity with a spherical surface diagram; students calculate flux on the worksheet.
6. Check for understanding (5 min): Exit ticket – one sentence explaining why field lines never cross.

Summarise how field‑line direction, density, and termination illustrate the nature of gravitational fields and link to quantitative concepts like flux. Collect exit tickets to gauge understanding, and assign a homework task to sketch field lines for a planet‑moon system and explain the density change with altitude.